Electric machines for use in motor vehicles have long been known in the form of the starter or starter motor and the generator. In modern vehicles, one may use alternating-current generators of claw pole configuration having electrical external excitation. For the rectification of the alternating current generated, rectifiers based on semiconductor diodes are normally used.
Because of the increasing requirements for electrical energy in the vehicle, the endeavor to reduce fuel consumption and lower emissions as well as the desire to combine the advantages of an electric motor with those of an internal combustion engine, electric machines having a double function, so-called starter-generators, may be used.
Starter-generators (SG) are electric machines that are able to be operated in a vehicle, according to requirement, as an electric motor or as a generator. As a generator, starter-generators have to be able to assume all the tasks which are conventionally assigned to the generator, namely, the electrical supply of the vehicle electrical system and charging the vehicle's battery. As an electric motor, at the start of the internal combustion engine, starter-generators have to bring its crankshaft to the required starting rotational speed in a short time.
The use of starter-generators is not, however, limited to the functions named. At a correspondingly large nominal capacity, a starter-generator, in motor operation, is able to support the internal combustion engine in driving, for instance, in accelerating in a so-called boost operation and for turbolag compensation. During braking, a part of the braking energy may be recuperated by a generator operation of a starter-generator, Corresponding drives are designated as hybrid drives, and corresponding systems as boost recuperation systems (BRS).
It would be desirable to improve the starting process of the internal combustion engine having such electric machines.